Metal halide discharge lamp having an arched arc tube

ABSTRACT

The arc tube of a metal halide arc discharge lamp has electrodes at each end thereof and contains a fill including mercury and a metal halide. The arc tube has an arch shape so that, during operation of the lamp in a horizontal position, the arc discharge is substantially equidistant from the walls of the arc tube.

United States Patent 1191 Koury 1451 Dec. 31, 1974 METAL HALIDE DISCHARGE LAMP HAVING AN ARCHED ARC TUBE [75] Inventor: Frederick Koury, Alexandria, NH.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

22 P116611 July 9,1973

21 Appl.No.:377,442

[52] US. Cl 313/220, 313/204, 313/228 [51] Int. Cl. H0lj 61/33 [58] Field of Search... 313/220, 204, 225, 227-229,

[56] References Cited UNITED STATES PATENTS 2,179,606 11/1939 Anderson, Jr. ..313/220 2/1940 Germer 313/220 X 2,298,239 10/1942 Stimkorb 313/204 2,965,790 12/1960 lttig et al. 313/225 x 3,753,019 8/1973 Hellman 313/229 x FOREIGN PATENTS OR APPLICATIONS 1,003,971 9/1965 Great Britain 313/227 Primary Examiner.lames W. Lawrence Assistant Examiner-Wm. H. Punter Attorney, Agent, or Firm.1ames Theodosopoulos [57] ABSTRACT The are tube of a metal halide arc discharge lamp has electrodes at each end thereof and contains a fill including mercury and a metal halide. The are tube has an arch shape so that, during operation of the lamp in a horizontal position, the arc discharge is substantially equidistant from the walls of the arc tube.

6 Claims, 1 Drawing Figure METAL I-IALIDE DISCHARGE LAMP HAVING AN ARCHED ARC TUBE BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to metal halide arc discharge lamps of the type used for general illumination. Such lamps have a generally cylindrical arc tube having electrodes at each end thereof. The arc tube contains a fill of mercury, metal halide and, for starting purposes, an inert gas. During normal operation, the pressure within the arc tube is between about one to ten atmospheres and the temperature of the arc tube is between about 500 to 1,000C.

2. Description Of The Prior Art Metal halide arc discharge lamps for general illumination have become commercially useful in the past five to ten years because they are more efficient and yield a whiter light than high pressure mercury vapor lamps. The arc tube in metal halide lamps is generally made of fused quartz (a high silica glass) and is generally a straight cylindrical tube having press seals at each end. Since the arc tube is a straight cylinder, positional orientation of the arc tube during lamp operation is immaterial.

An example of a metal halide arc discharge lamp is shown in US. Pat. No. 3,761,758 issued Sept. 25, 1973. Disclosed in said patent is a list of about two dozen patents generally relating to metal halide arc discharge lamps of the type with which this invention is concerned.

Although there is no prior art that discloses arched or curved arc tubes in the field of metal halide electroded arc discharge lamps, other types of are discharge lamps have employed curved arc tubes. For example, arc discharge mercury lamps of 50 or 60 years ago did use curved arc tubes, but the reason for curving the arc tube was to keep apart the two pools of liquid mercury that served as electrodes.

Some capillary arc discharge lamps of thirty or forty years ago did have only the upper arc tube wall of a horizontal operation lamp curved to prevent overheating of the middle of the arc tube. Such lamps were very heavily loaded (watts/sq.cm.) in comparison to presently used commercial metal halide lamps and had a thicker arc tube wall and smaller bore. In such lamps, the arc discharge extended to the walls of the arc tube and was confined thereby; in contrast, the arc discharge of present metal halide lamps is not confined by the walls of the arc tube and the arc discharge shape cna, therefore, be affected by convection currents within the arc tube.

Curved arc tubes are also disclosed in the field of low pressure are discharge lamps, such as fluorescent lamps and low pressure sodium lamps. It is common for such lamps to employ a U-shaped arc tube in order to conserve space, for the reason that such arc tubes are so lightly loaded that an extremely long arc tube is needed, in comparison to metal halide lamps, for a lamp of even moderate wattage.

SUMMARY OF THE INVENTION In present commercial metal halide arc discharge lamps used in horizontal operation, the arc discharge is affected by convection currents in the arc tube during normal operation, so that the arc discharge is not coaxial with the straight cylindrical arc tube. Instead, the arc discharge is bowed or raised to the upper portion of the arc tube and extends to the wall thereat. I have discovered that when the arc tube is arched, so that an arc discharge, bowed by convection currents during normal horizontal lamp operation, is substantially centered within the arc tube, there is a substantial unexpected increase in the efficiency of the lamp. The in crease is about 10 or 15 percent and may be higher. Centering of the arc discharge within the arc tube means that the core of the arc discharge substantially coincides with the curved axis or center line of the arch shaped arc tube or, in other words, that the visible arc discharge is substantially equidistant from the walls of the arc tube.

It is necessary, of course, that a lamp in accordance with this invention have a base that positions the arc tube with its arch uppermost when the lamp is inserted in a socket. This is in contrast to the usual screw type base that is used with prior art commercial metal halide lamps; a screw type base is not a positioning base and can be used in prior art lamps since the arc tubes thereof are straight cylinders.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE in the drawing is an elevational view of a metal halide arc discharge lamp having an arched arc tube in accordance with this invention; the view shows the lamp in a horizontal position with the arch uppermost.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing, an arc discharge lamp in accordance with this invention comprises an outer glass envelope or jacket I. Jacket I is provided at its end with a sealed reentrant stem 2 through which extend relatively stiff lead-in wires 3 and 4 connected at their outer ends to the electrical contacts of a positioningtype base 5. Disposed within jacket 1 is an arched arc tube 6. The relationship between arc tube 6 and base 5 is such that when base 5 is inserted into a suitable socket, the lamp will be in a substantially horizontal position, with the arch of arc tube 6 uppermost.

Arc tube 6 is supported within jacket I by means of metal frames 7 and 8 at each end of arc tube 6. Metal frames 7 and 8 comprise rigid wires 9 and 10, respectively, to which are fastened clamps 11 and 12 each of which supports a pressed seal end of arc tube 6. Clamps l1 and 12 are located angularly in respect to the axis of envelope 1, as shown in the drawing, because of the arched shape of arc tube 6. In the prior art, similar type clamps were generally perpendicular to the envelope axis, since the arc tube was coaxial with the envelope.

Metal frame 7 is supported by lead-in wire 4 to which it is welded. Metal frame 8 is supported at the other end by metal leaf springs 13 which press against the inner wall of envelope 1.

Electrical connection from lead-in wire 4 to the proximate main solid electrode (not shown; it is hidden by reflective coating 14) is through connective wire 15. Electrical connection from lead-in wire 3 to the other main electrode (also not shown) is through wire 16. Electrical connection from lead-in wire 3 to the starter electrode (not shown) is through resistor 17. Bimetal switch 18 shorts the starter electrode to the adjacent main electrode after lamp ignition occurs. 7

The atmosphere within jacket 1 is an inert gas, such as nitrogen; also disposed within jacket 1 is a getter 19, for gettering hydrogen, mounted on frame 8.

Arc tube 6 has an arch shape that substantially conforms to the shape of the arc discharge therein during normal operation of the lamp in a horizontal position. During such operation the arc discharge normally bows upward primarily because of convection currents within the arc tube. Optimum results for the purpose of this invention are obtained when the arc discharge is substantially centered within the arched arc tube, that is to say, when the visible arc discharge is substantially equidistant from the walls of the arc tube.

In a comparison of 400 watt metal halide lamps, prior art lamps, that is, lamps having a straight arc tube had efficiencies of 85 to- 87 lumens per watt during horizontal operation. Lamps in accordance with this invention, that is, lamps having an arched arc tube had efficiencies of 102 to 105 lumens per watt. In the prior art lamps, the visible arc discharge was pushed against the upper wall of the arc tube for almost the entire length of the arc tube while the space between the arc discharge and the lower wall was equal to about half the diameter of the visible arc discharge.

In the lamps of this invention, the visible are discharge was substantially centered within the arched arc tube, the space between the discharge and the walls of the arc tube equalling about one-quarter of the diameter of the visible arc discharge. In these are tubes the curvature was such that the angle formed by the two ends of the arc tube was about 140.

Experiments on varying the curvature so that the angle formed by the two ends of the arc tube varied all the way from 180 (a straight tube) to (a U-shaped tube) showed that centering of the arc discharge generally occurred at angles between about 130 and 160, at least for metal halide lamps of the type presently used for general illumination. The amount of bowing of the arc discharge and, consequently, the shape of the arched arc tube, is primarily dependent on the velocity of the convection currents within the arc tube which, in turn, depends on the pressure within the arc tube, the temperature of the discharge and the diameter of the arc tube. The amount of bowing of the are discharge increases with increasing pressure, increasing temperature and increasing arc tube diameter.

Another unexpected advantage of lamps made in accordance with this invention is a significant improvement in maintenance during lamp operation. In the 400 watt lamps previously mentioned, the prior art lamps had a maintenance of 72.5 percent after 2,000 hours operation. That is to say, their efficiency in lumens per watt after 2,000 hours was only 72.5 percent of their initial efficiency. In the 400 watt lamps as per this invention, the maintenance after 2,000 hours operation was 87 percent.

I claim:

1. A metal halide arc discharge lamp comprising: an outer envelope; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, said are tube containing a fill including mercury, metal halide and an inert gas, the arched shape of said are tube being such that during normal horizontal operation with the arch of the arc tube uppermost, the are discharge does not extend to the walls of the arc tube and said discharge is substantially equidistant from the walls of the arc tube; and means to energize said arc tube.

2. A metal halide arc discharge lamp comprising: an outer envelope having a positioning type base at one end thereof; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said are tube being such that during normal horizontal operation of the lamp with the arch of the arc tube uppermost, the arc discharge is spaced from and is substantially centered within the arc tube.

3. A metal halide arc discharge lamp comprising: an outer envelope having a positioning type base at one end thereof; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said arc tube being such that during normal horizontal operation of the lamp with the arch of the arc tube uppermost, the arc discharge does not extend to the walls of the arc tube and the space between said discharge and said walls is substantially uniform around the circumference of said discharge.

4. In a general illumination metal halide are discharge lamp of the type wherein the arc discharge is bowed upward by convection currents within the arc tube during normal lamp operation, the improvement which comprises an envelope having a base at one end thereof and an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said arc tube being such that during normal horizontal operation with the arch of the arc tube uppermost, the arc discharge does not extend to the walls of the arc tube and is substantially centered therein.

5. The lamp of claim 4 wherein the angle formed by the two ends of the arc tube is between about and 6. The lamp of claim 4 wherein, during normal lamp operation, the core of the arc discharge substantially coincides with the longitudinal center line of the arched arc tube. 

1. A metal halide arc discharge lamp comprising: an outer envelope; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, said arc tube containing a fill including mercury, metal halide and an inert gas, the arched shape of said arc tube being such that during normal horizontal operation with the arch of the arc tube uppermost, the arc discharge does not extend to the walls of the arc tube and said discharge is substantially equidistant from the walls of the arc tube; and means to energize said arc tube.
 2. A metal halide arc discharge lamp comprising: an outer envelope having a positioning type base at one end thereof; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said arc tube being such that during normal horizontal operation of the lamp with the arch of the arc tube uppermost, the arc discharge is spaced from and is substantially centered within the arc tube.
 3. A metal halide arc discharge lamp comprising: an outer envelope having a positioning type base at one end thereof; an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said arc tube being such that during normal horizontal operation of the lamp with the arch of the arc tube uppermost, the arc discharge does not extend to the walls of the arc tube and the space between said discharge and said walls is substantially uniform around the circumference of said dischargE.
 4. In a general illumination metal halide arc discharge lamp of the type wherein the arc discharge is bowed upward by convection currents within the arc tube during normal lamp operation, the improvement which comprises an envelope having a base at one end thereof and an arched arc tube, having electrodes at each end thereof, disposed within said envelope, the arched shape of said arc tube being such that during normal horizontal operation with the arch of the arc tube uppermost, the arc discharge does not extend to the walls of the arc tube and is substantially centered therein.
 5. The lamp of claim 4 wherein the angle formed by the two ends of the arc tube is between about 130* and 160*.
 6. The lamp of claim 4 wherein, during normal lamp operation, the core of the arc discharge substantially coincides with the longitudinal center line of the arched arc tube. 